dentinal tubule occlusion using er:yag laser: an in vitro
TRANSCRIPT
J Appl Oral Sci.
Abstract
Submitted: July 17, 2020Modification: September 19, 2020
Accepted: October 15, 2020
Dentinal tubule occlusion using Er:YAG Laser: an in vitro study
Objectives: We analyzed the effects of the Er:YAG laser used with different parameters on dentinal tubule (DT) occlusion, intrapulpal temperature and pulp tissue morphology in order to determine the optimal parameters for treating dentin hypersensitivity. Methodology: Dentin specimens prepared from 36 extracted human third molars were randomized into six groups according to the treatment method (n=6 each): control (A); Gluma desensitizer (B); and Er:YAG laser treatment at 0.5 W , 167 J/cm2 (50 mJ, 10 Hz) (C), 1 W , 334 J/cm2 (50 mJ, 20 Hz) (D), 2 W , 668 J/cm2 (100 mJ, 20 Hz) (E), and 4 W and 1336 J/cm2 (200 mJ, 20 Hz) (F). Treatment-induced morphological changes of the dentin surfaces were assessed using scanning electron microscopy (SEM) to find parameters showing optimal dentin tubule occluding efficacy. To further verify the safety of these parameters (0.5 W, 167 J/cm2), intrapulpal temperature changes were recorded during laser irradiation, and morphological alterations of the dental pulp tissue were observed with an upright microscope. Results: Er:YAG laser irradiation at 0.5 W (167 J/cm2) were found to be superior in DT occlusion, with an exposure rate significantly lower than those in the other groups (P<0.05). Intrapulpal temperature changes induced by Er:YAG laser irradiation at 0.5 W (167 J/cm2) with (G) and without (H) water and air cooling were demonstrated to be below the threshold. Also, no significant morphological alterations of the pulp and odontoblasts were observed after irradiation. Conclusion: Therefore, 0.5 W (167 J/cm2) is a suitable parameter for Er:YAG laser to occlude DTs, and it is safe to the pulp tissue.
Keywords: Er:YAG laser. Dentin hypersensitivity. Power. Scanning electron microscopy. Temperature.
Hongmin ZHUANG1,2#
Yuee LIANG1#
Shaowen XIANG3
Huanying LI1
Xingzhu DAI1
Wanghong ZHAO1
Original Articlehttp://dx.doi.org/10.1590/1678-7757-2020-0266
1Southern Medical University, Nanfang Hospital, Department of Stomatology, Guangzhou, Guangdong, China.2Guangzhou Hospital of Integrated Traditional and West Medicine, Department of Stomatology, Guangzhou, China.3Southern Medical University, Stomatological Hospital, Guangzhou, Guangdong, China.
#Hongmin Zhuang and Yuee Liang contributed equally to this work.
Corresponding address:Wanghong Zhao - Department of Stomatology
- Nanfang Hospital of Southern Medical University - 510000 - Guangzhou - China.
Phone: 020-62787680e-mail: [email protected]
2021;29:e202002661/10
J Appl Oral Sci. 2021;29:e202002662/10
Introduction
Dentin hypersensitivity (DH) is one of the
most frequently encountered chronic conditions
characterized by transient and sharp tooth pain evoked
by external stimuli, including thermal, evaporative,
tactile, osmotic, and chemical stimuli. The discomfort
caused by DH cannot be ascribed to any other dental
defect or pathology.1 According to Splieth and Tachou,
et al.2 (2013) 3%–98% of individuals are affected
by DH, which can cause varying degrees of irritation
during eating, drinking, and even breathing.
Although the DH mechanism remains controversial,
the theory of hydrodynamics is the most accepted.
It suggests that external stimulation of teeth with
DH results in fluid displacement within the dentinal
tubules (DTs),3 which activates the nerve endings
located at the pulp–dentin interface and eventually
results in pain and discomfort. According to the
theory of hydrodynamics, DT narrowing or occlusion
for minimizing dentin permeability and lowering the
pulp sensitivity threshold is a potential strategy for
pain relief. Frequently used desensitizing agents can
be classified into four categories: anti-inflammatory
agents (corticosteroids), protein precipitants
(formaldehyde, silver nitrate, strontium chloride
hexahydrate), tubule-occluding agents (calcium
hydroxide, potassium nitrate, sodium fluoride), and
tubule sealants (resins and adhesives).4 However, none
of these agents can produce long-lasting effects, since
abrasion and erosion by internal and external acids
would lead to re-exposure of DTs over time.5
The advent of laser treatment has provided an
alternative modality for DH management.6 Currently
used lasers for this purpose include Nd:YAG lasers,
Er:YAG lasers, Er,Cr:YSGG lasers, carbon dioxide
lasers, and diode lasers.7,9 Among these, Er:YAG lasers
with a wavelength at 2940 nm exhibit high absorption
in water and are expected to minimize thermal damage
to the pulp and dentin tissues.10 Walsh and Cummings11
(1994) found that water absorption was 15 and 10,000
times greater with Er:YAG lasers than with CO2 and
Nd:YAG lasers, respectively. Therefore, due to the high
water absorption peak compared to other commercially
available lasers, Er:YAG lasers have gained popularity
in clinical settings for treating oral diseases after it was
approved by the U.S. Food and Drug Administration
in 1997.12 When it comes to clinical practice in
treating DH, parameters vary between brands due to
differences in setups (Table 1). However, few studies
have evaluated the optimal parameters for the Er:YAG
laser in terms of DH treatment.13-14
In this study, we assumed the Er:YAG laser
with optimal parameters can effectively treat DH
by occluding DT and had no damage to the dental
pulp. Therefore, the objective of this in vitro study
is to explore the parameters of the Er:YAG laser
when used for dentinal tubule occlusion to provide
guidance for the clinical treatment of DH. As such,
we investigated the effects of laser irradiation using
these parameters on intrapulpal temperature changes
and the morphological alterations in odontoblasts and
pulp tissue were observed to determine the safety of
Er:YAG laser in the treatment of DH.
Brands Power (W) Frequency (Hz)
Energy/Pulse(mJ)
Irridiation Time(s)
Irridiation Area(mm2)
Distance (mm)
Applicator device
Water spray
Erwin - 10 40 47 4 Slight contact - yes
DE- Light - 30 60 10 - 3~4 Straight quartz round tip
no
Smart 2940 d plus, Deka
- 10 100 60 - 1 R14 handpiece yes
Fidelis III,Fotona 0,2 3 80 120 7,74 6 R02-handpiece
yes
Fidelis Plus,Fotona
- 30 100 60s/cm2 - 1~2 R07-handpiece
yes
Key laser 3+, KaVo
- 2 40 40 4 25 2060 handpiece
no
Key Laser 1243,KaVo
- 2 60 Four irradiation of 20s, with a 1-min interval
9 6 2055 handpiece
No, only air cooling
Table 1- Different Parameters of Er:YAG Laser of Different Brands Treating Dentin Hypersensitivity
Dentinal tubule occlusion using Er:YAG Laser: an in vitro study
J Appl Oral Sci. 2021;29:e202002663/10
Methodology
Study designAn in vitro study was conducted and the study
protocol (Figure 1) was reviewed and approved by the hospital’s Institutional Review Board with a reference
number NFEC-201701-K1-01.
Preparation of dentin specimensHuman third molars extracted from adults aged
20–25 years old were thoroughly cleaned and inspected
under magnification (×20). Those with cracks, caries,
and restorations were discarded. Eventually, 36 molars
were selected. Dentin specimens (DSs) with 2 mm
thickness and 3×3 mm2 area were prepared from all
36 teeth using a high-speed diamond bur (Mani Inc.,
Japan) under water irrigation. In a direction parallel to
the occlusal surface, enamel was removed up to 2 mm
below the central fossa so that dentin was exposed. For
homogeneous dentin surfaces, 200-, 600-, and 800-
grit silicon carbide papers (SUISUN Ltd., HK, China)
were used for polishing the specimens, which were
then washed with a large amount of distilled water
and disinfected by storage in distilled water with 0.2%
thymol (ZhiYuan Ltd., Tianjin, China) for no more
than 1 week until further use. Before the experiment,
all specimens were conditioned with 35% phosphoric
acid for 1 min (3M ESPE, St Louis, MN, USA) for DT
exposure.
Er:YAG laser TreatmentFollowing dentin exposure, the teeth were divided
into six groups of six teeth each (according to random
number table). Group A (control group) received no
further treatment after exposure to 35% phosphoric
acid. In group B, Gluma desensitizer (GD; Heraeus,
Germany) was gently applied using cotton pellets, and
the treated specimens were set aside for 60 s. Then,
they were dried until the dentin surfaces lost their
shine and subsequently rinsed with distilled water. The
same procedure was performed twice. The specimens
in groups C–F received Lite Touch Er:YAG laser (Lite
Figure 1- Flow diagram of the study
ZHUANG H, LIANG Y, XIANG S, LI H, DAI X, ZHAO W
J Appl Oral Sci. 2021;29:e202002664/10
Touch, Syneron Medical Ltd., Israel) irradiation at a
wavelength of 2490 nm under the following sets of
parameters: group C, 0.5 W, 167 J/cm2 (50 mJ, 10 Hz);
group D, 1 W, 334 J/cm2 (50 mJ, 20 Hz); group E, 2 W,
668 J/cm2 (100 mJ, 20 Hz); and group F, 4 W, 1336 J/
cm2 (200 mJ, 20 Hz). The other conditions remained
the same for all groups (Table 2). Laser energy was
delivered via the Magnum tip (green O-rings; length:
6.3 mm, diameter: 1.3 mm), which was placed at a
1-cm distance, under a water spray at level 1 for 30 s.
During irradiation, the tip was moved to a mesiodistal
direction at a speed of approximately 1 mm/s, and
the irradiation area of specimens was 3*3 mm2. All
irradiation procedures were performed by a single
researcher to ensure treatment of the entire dentin
surface with minimum variations.
SEM observationThe treated specimens were fixed in 2.5%
glutaraldehyde (Phygene Com., Fuzhou, China) for 24
h at room temperature, rinsed with 0.1 M phosphate-
buffered saline for glutaraldehyde removal, and
air-dried. Then, they were dehydrated in a series of
alcohol solutions (ZhiYuan Ltd., Tianjin, China) (30%,
50%, 70%, 80%, 95%, 100%; 15 min for each),
sputter-coated with a layer of gold, and observed
under a scanning electron microscope (S-4800 SEM,
Hitachi Ltd., Hitachinaka, Japan) at 1500× and 5000×
magnification.
The area of open or partially obliterated DTs
observed by scanning electron microscopy (SEM) was
measured by software (Image-Pro PLUS 6.0, Media
Cybernetics, USA). On the basis of pixel grey value
differences, the software can differentiate these DTs
by drawing their outlines, thus facilitating calculation
of the area of open or partially obliterated DTs. The
tubule exposure rate for each group was subsequently
calculated using the following formula:
Intrapulpal temperature measurementsIn the preceding experiments, the surface of dental
specimens treated with parameters 0.5 W, 167 J/cm2
(50 mJ, 10 Hz) has shown the most desirable structural
changes without microcracks and carbonization, so we
chose these parameters for the follow-up experiments.
Twelve freshly extracted third molars were prepared
and irradiated with parameters of 0.5 W, 167 J/cm2
(50 mJ, 10 Hz).
Before being irradiation, enamel was removed
up to 2 mm in depth below the central fossa in a
direction parallel to the occlusal surface so that dentin
was just exposed. A diamond bur was used to mark
an irradiation area of 3×3 mm2 at the center of the
dentin. A hole with a diameter of 1 mm was created
subjacent to the dentinoenamel junction to create
access for the insertion of a type K thermocouple
(diameter: 1 mm) into the pulp chamber. The type K
thermocouple was connected to a digital thermometer
(DT-610B, CEM, China). A thermal paste (TaoXin
Com., Shenzhen, China) was introduced into the pulp
chamber to ensure good contact between the tip of the
thermocouple and the ceiling of the chamber (Figure
2). The heat conductivity of this paste was similar to
that of the dental pulp. The root was sealed by glass
ionomer cement (GC Fuji IX, Tokyo). After inserting
the thermal paste and thermocouple, the cervical hole
was sealed with wax.
(exposure rate = mean total area of open or partially obliterated DTs) mean total area
Groups N Energy(mJ) Frequency(Hz) Power(W) Energy density(J/cm2)
Group C 6 50 10 0,5 167
Group D 6 50 20 1 334
Group E 6 100 20 2 668
Group F 6 200 20 4 1336
Table 2- Parameters of Laser Groups
Figure 2- Schematic diagram of detection of temperature changes in pulp chamber
Dentinal tubule occlusion using Er:YAG Laser: an in vitro study
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Irradiation was performed with (group G; n=6) and
without (group H; n=6) air and water cooling, while
other conditions remained the same as those described
earlier. Temperature changes during irradiation were
recorded at 5 or 10 s intervals by calculating the
difference between the recorded values and the initial
temperature values.
Morphological alterations of pulp tissueTwelve healthy third human molars were selected to
remove coronal enamel to just expose dentin beneath,
yielding twelve dentin specimens. They were divided
randomly into 2 groups, as laser group (group A, 0.5
W, 167 J/cm2) and control group (group B). Following
our previous outcomes, the laser group was applied
with a treatment using parameters of 0.5 W, 167 J/
cm2, while the control group was treated with nothing.
They were cut longitudinally to take the pulp tissue.
HE (hematoxylin-eosin) staining was used to observe
pulp histomorphology by light microscopy (Olympus
BX51; Olympus Optical Co., Ltd., Tokyo, Japan).
Statistical analysisAll collected data were statistically analyzed using
SPSS version 23.0 (SPSS Inc., Chicago, Illinois, USA).
Multiple intergroup comparisons were performed using
the Kruskal–Wallis test. When this test presented a
significant difference, the multiple (double) comparison
Mann–Whitney U test was used. A p-value of <0.05
was considered statistically significant.
Results
SEM observationSEM images for the control group showed
numerous exposed DTs parallel to each other, without
plugged debris (Figure 3: A, a). The micrograph for
the Gluma desensitizer group revealed the occlusion
of several DTs by precipitant plugs, with a few partially
occluded DTs (Figure 3: B, b). In group C (0.5 W, 167
J/cm2), a thick, smooth, melted layer covering the
Figure 3- SEM Micrographs of treated dental specimens of group A (A,a; ×1500,×5000), group B (B,b; ×1500,×5000), group C (C,c; ×1500,×5000), group D (D,d; ×1500,×5000), group E (E,e; ×1500,×5000), group F (F,f; ×1500,×5000)
ZHUANG H, LIANG Y, XIANG S, LI H, DAI X, ZHAO W
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superficial dentin surface was observed (Figure 3: C,
c). The DTs were almost completely obliterated by this
layer. In group D (1 w, 334 J/cm2), the dentin surface
appeared to be melting with the formation of bubbles,
and a few partially occluded DTs were observed (Figure
3: D, d). The other two groups (groups E and F),
which involved the use of stronger powers, revealed
very similar, scale-like surfaces with open tubules of
different depths (Figure 3: E, e, F, f).
Figure 4 shows comparisons of the exposure rates
between groups. There were significant differences
among all six groups (P<0.001). The tubule exposure
rate of the Gluma desensitizer treatment group (group
B) was significantly lower than that of the control
group (P<0.05), but still higher than the exposure
rates of groups C and D (P<0.05). In laser groups,
the exposure rate in group C (0.0002±0.0002) was
significantly lower than that of other groups (P<0.05),
and the exposure rate evidently increased with an
increase in power (P<0.05).
Intrapulpal temperature measurementsOn the basis of the favorable results obtained
for group C in the preceding experiments, we used
the irradiation parameters of 0.5 W and 167 J/cm2
for intrapulpal temperature measurements. Figure
4 shows the results of the intrapulpal temperature
measurements during Er:YAG laser irradiation at 0.5
W and 167 J/cm2. Under air and water cooling, the
final temperatures were lower than the temperature
registered before irradiation. The temperature
gradually decreased by −2.275±0.597°C and
gradually increased thereafter, with a change of
−1.725°C ± 0.359°C recorded at 190 s. In contrast,
laser irradiation without air and water cooling for 60 s Figure 4- Comparisons of Exposure rates of tubules. a–f indicate statistically significant differences between groups (P<0.05)
Figure 5- Intrapulpal temperature change during Er:YAG laser irradiation (0.5 W , 167 J/cm2) with and without air and water spray cooling
Dentinal tubule occlusion using Er:YAG Laser: an in vitro study
J Appl Oral Sci. 2021;29:e202002667/10
resulted in a temperature change of 5.067°C±0.058°C
(Figure 5).
Intrapulpal temperature change during Er:YAG
laser irradiation (0.5 W , 167 J/cm2) with and without
air and water spray cooling.
Morphological alterations of pulp tissueThe morphology of the pulp was observed by
light microscopy after HE staining. No significant
differences were observed between the two groups.
The morphology of the odontoblast cells and vessels,
as well as of the collagenic and neural fibers, was clear
and healthy (Figure 6).
Discussion
In this in vitro study, Er:YAG laser with the
parameters of 0.5 W, 167 J/cm2 (50 mJ, 10 Hz)
under a water spray at level 1 was effective in
occluding dentin tubules and harmless to the dental
pulp, which provided the theoretical basis for the
treatment of dentin hypersensitivity.
Absi, Addy and Adams15 (1987) showed a number
of open DTs per surface area eight times greater in
teeth with DH than in those without DH, and the
tubular diameter was two times greater in sensitive
teeth than in insensitive teeth. Moreover, there was
a comparative study suggesting that 35% phosphoric
acid resulted in better DT exposure than 24%
ethylenediamine tetraacetic acid (EDTA) did when
under SEM.16 Therefore, in the present study, DTs
in DSs were exposed to 1-min application of 35%
phosphoric acid to establish DH models. SEM images
for our control group showed clean and smooth dentin
surfaces with tubule orifices that were free of smear
layers and plugs; these findings were consistent with
those of previous studies. 10,12,14
Gluma desensitizer is composed of glutaraldehyde
and 2-hydroxyethyl-methacrylate (HEMA), which
coagulates the serum albumin in dentinal fluid. This
reaction between glutaraldehyde and albumin induces
HEMA polymerization.17,18 Thus, the desensitizer can
form a coagulation plug similar to the melted layer
formed after laser irradiation. We used the gluma
desensitizer as a positive control in the present study,
in accordance with several other studies.19,20 There
is lack of consensus over whether laser serves as a
better option in treating DH than Gluma desensitizer.
An 18-month randomized clinical study conducted
by Lopes, Euardo e Aranha21 (2017) showed that
compared to the Nd:YAG laser treatment group and
the Nd:YAG laser+Gluma desensitizer treatment
group, the Gluma desensitizer treatment group
had the most prolonged duration on desensitizing.
However, Ozlem, et al.1 (2018) used the yeaple probe
to evaluate the dentin sensitivity of patients with
dentin hypersensitivity treated by Er:Cr:YSGG laser or
Gluma desensitizer or a combination of the two. The
results showed that using Er:Cr:YSGG laser to treat
the disease alone could get the most desirable results
even at different time intervals (7, 90, 180 days).1,21
Considering that the wavelength of Er:YAG laser is
closed to that of the Er:Cr:YSGG laser, the principle
of action of the two lasers in occluding dentin tubules
is similar. The excellent efficacy of Er:Cr:YSGG laser
could serve as a solid foundation for the promising
application prospects of Er:YAG laser in treating DH.
Er:YAG lasers are high-power lasers, and we used
powers of 0.5 (lowest) to 4 W in the present study.
According to Table 1, the Er:YAG laser parameter
settings for desensitization treatment are usually low
(the output power range is between 0.08 W-3 W) and,
as for the application of the cooling system, when
the output power is high (3 W), the laser irradiation
should be accompanied by water, whereas when the
output power is low (0.08 W), laser irradiation could
Figure 6- Micrograph of pulp tissue after HE staining ×200. (A: control group; B: laser group, 0.5 W , 167 J/cm2)
ZHUANG H, LIANG Y, XIANG S, LI H, DAI X, ZHAO W
J Appl Oral Sci. 2021;29:e202002668/10
work without water. Considering that the lowest built-
in parameter of the laser used in this experiment is
set to 0.5 W, we set 0.5W as the starting value for
parameter exploration, and at the same time turned
on the water-air mode for safety reasons. The laser-
treated groups exhibited significant differences in
SEM findings. Moreover, the DT exposure rate was the
lowest after irradiation at 0.5 W and 167 J/cm2, with
the specimens showing almost complete DT occlusion
by the melted layer on SEM images. Our findings
were consistent with the findings of previous studies
exploring the DT occluding effects of the Er:YAG laser,
although the parameters used in the present study
were different from those used in previous studies.9
Belal and Yassin12 (2014) evaluated the effects of an
Er:YAG laser on DT occlusion using SEM to observe
melted areas around exposed DTs. The percentage of
occluded tubules was found to be significantly greater
in the Er:YAG laser group than in the other groups.
Moreover, Badran et al.22 (2011) reported that 120
s of Er:YAG laser irradiation could lead to complete
DT occlusion, showing a wrinkled, melted dentin
surface with no visible signs of DTs. In a study of
Belal and Yassin12 (2014), the laser power (40 mJ, 10
Hz) is slightly lower than that in this study, while the
irradiation distance is shorter (the study of Belal and
Yassin12): slight contact; this study: 30mm). Similarly,
the parameter setting in a study by Badran et al.22
(2011) is 60 mJ, 2 Hz, (0.12 W), significantly lower
than 0.5 W used in this study, but the irradiation time
(60 s) is twice the time of 30 s, and there is no water
irrigation, which clearly enhances the melting effect
of the Er:YAG laser. Overall, the thermomechanical
ablation of Er:YAG laser may be a major influencing
factor for controlling application parameters of the
laser. Temperature increase on the irradiated surface
can induce melt and recrystallization of the dentin
tissue, resulting in obliteration of the tubule orifices.8
Interestingly, we found that the tubule exposure
rate increased as the power setting of the laser device
increased. In comparison with the dentin surface
treated at 0.5 W, 167 J/cm2, treatment at 1 W, 334 J/
cm2 exhibited melting with a bubble-like appearance
and a few partially occluded DTs. Our findings were
in accordance with those of another study,23 and this
phenomenon can be attributed to the fact that higher
power settings may result in rapid water evaporation
instead of DT occlusion; the rapid water evaporation
results in microexplosions on the irradiated surface,
which cause such morphological alterations.24 Further,
dentin treated at 2 W and 668 J/cm2 and dentin treated
at 4 W and 1336 J/cm2 exhibited a similar appearance
with a significant difference in the tubule exposure rate
(P <0.05). We speculated that the similar stripped
surfaces were caused by the cutting of superficial hard
tissues when the laser power exceeded the ablation
threshold. Other studies also showed similar results.
Harashima, et al.25 (2005) compared morphological
features between cavities prepared by an Er:YAG laser
and those prepared by an Er,Cr:YSGG laser and found
similar, irregular, rugged surfaces with open DTs in
both groups. At a wavelength of 2940 nm, the energy
of Er:YAG lasers is more strongly absorbed by water
than by hard tissues,26,27 resulting in microexpansion
that can produce hydrokinetic forces for clear and
quick removal of the target hard tissue via mechanical
separation.28
Therefore, based on the SEM images, the
parameters 0.5 W and 167 J/cm2 seem to be suitable
for adequate DT occlusion. Nevertheless, energy
accumulation from laser treatment may cause damage
to the pulp tissue health. Studies have shown that the
pulp would respond to externally applied heat.29,30 An
intrapulpal temperature increase of 5.5°C could result
in necrosis of 15% dental pulp, whereas when the
temperature increased by 11°C, pulpal necrosis could
occur in 60% of the pulp.
In the present study, Er:YAG laser irradiation at 0.5
W and 167 J/cm2 under water and air cooling initially
induced a decrease in the intrapulpal temperature
(−2.275°C±0.597°C). Similarly, Yaneva et al.31 (2016)
investigated temperature changes in the pulp chamber
during root planing using the Er:YAG laser and found
temperature decreases of 1.6°C, 2.4°C, 2.5°C, and
2.5°C after every 10 s. Intrapulpal temperature
changes depend on the following factors: the laser
emission technique (pulsed or continuous), distance
between the applicator and target tissue, wavelength
of the laser beam, use of air or water cooling during
irradiation, duration of irradiation, and movement
of the handpiece.31 Thanks to the wavelength,
Er:YAG lasers are characterized by a high absorption
coefficient, which indicates shallow tissue penetration
for both hard and soft tissues.32 Therefore, Er:YAG
lasers are unlikely to cause adverse thermal effects
in tissues. Moreover, pulsed emission of the laser
beam can, to some extent, allow for the normalization
of the temperature of the irradiated tissue before
Dentinal tubule occlusion using Er:YAG Laser: an in vitro study
J Appl Oral Sci. 2021;29:e202002669/10
irradiation by the next laser beam. At the same time,
the importance of continuous water and air cooling
during irradiation, which prevents an obvious increase
in the intrapulpal temperature physically, should not
be neglected. Collectively, all the factors described
above contributed to the intrapulpal temperature
decrease in the air and water cooling group. However,
the temperature gradually increased over time, with
a change of −1.725°C±0.359°C recorded at 190 s.
This indicates that the duration of irradiation is also
an important factor for pulp safety. In a previous
study, the intrapulpal temperature change within
30 s of Er:YAG laser irradiation under air and water
cooling was recorded as −2.2°C±1.5°C. Moreover,
intrapulpal temperature gradually increased with
longer duration of irradiation, which corroborates the
findings of the present study.33 We found that the
intrapulpal temperature increase during irradiation
without water and air cooling was 1.833°C±0.473°C
at 30 s and 5.067°C±0.058°C at 60 s, and the final
increase was lower than the safe threshold of 5.6°C
reported by Zach and Cohen30 (1965). Collectively,
although water and air cooling during laser irradiation
has been demonstrated to be important for pulp safety,
the parameters in this study (0.5 W, 167 J/cm²) enjoy
a highly safety even without cooling.
As for morphological alterations of the pulp
tissue, no significant morphological alteration of
the odontoblasts was found after treatment with
0.5 W (167 J/cm²), according to HE staining. Thus,
parametersof 0.5 W (167 J/cm²) could be safe for
Er:YAG laser treatment for DH.
In summary, we conducted a preliminary in vitro
study investigating suitable parameters for the
successful treatment of DH using the Er:YAG laser. Our
findings can, to some extent, serve as a reference for
further clinical trials. Taking the high water absorption
of Er:YAG laser energy into account, the fluid in teeth
and the blood circulation in the pulp may reduce the
increase in temperature, consequently increasing
the safety of parameters in actual clinical trials.
However, this study has several limitations: first, the
sample size was relatively small. Second, it is an in
vitro study, and hence clinical trials with long-term
follow-up examinations under intraoral conditions like
brushing and acidic challenges are required. Third,
it was very difficult to standardize the variations of
the DT numbers of dentin even at same depth bellow
the dentin due to individual variations. In addition,
the pulpal response to this treatment also requires
in-depth investigations to further verify its practical
safety.
Conclusions
Our findings suggest that Er:YAG laser irradiation
at 0.5 W and 167 J/cm2 under a water spray at level
1 can effectively occlude DTs without any adverse
thermal effects on the pulp.
FundingsThis work was supported by the Science and
Technology Department of Guangdong Province
of China (No. 2018B030311047), the Science and
Technology Program of Guangzhou, China (No.
201804010419), and the President Foundation of
Nanfang Hospital, Southern Medical University (No.
2017C005).
AcknowledgementsThe authors thank Guangzhou Institute of Energy
Conversion, Chinese Academy of Sciences for
assistance with our SEM analyses. We would like to
thank Editage [www.editage.cn] for English language
editing.
Author disclosure statementThe authors declare no financial conflicts of
interest.
Authors' contributions
Zhuang, Hongmin: Conceptualization (Lead);
Data curation (Lead); Formal analysis (Lead);
Investigation (Lead); Methodology (Equal); Resources
(Lead); Validation (Lead); Visualization (Lead); Writing:
original draft (Lead). Liang, Yuee: Conceptualization
(Equal); Formal analysis (Equal); Methodology (Equal);
Project administration (Equal); Resources (Equal);
Writing: review & editing (Equal). Xiang, Shaowen: Writing: review & editing (Equal). Li, Huanying: Data curation (Equal). Dai, Xingzhu: Formal analysis
(Equal). Zhao, Wanghong: Conceptualization
(Equal); Writing: review & editing (Equal).
ZHUANG H, LIANG Y, XIANG S, LI H, DAI X, ZHAO W
J Appl Oral Sci. 2021;29:e2020026610/10
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Dentinal tubule occlusion using Er:YAG Laser: an in vitro study